Thin film waveguide with a periodically modulated nonlinear optical coefficient

ABSTRACT

A phase matched thin film waveguide supportive of electromagnetic wave energy having angular frequencies is disclosed. In one embodiment, the waveguide includes a sequence of strips each of length A/2 in the direction of wave energy propagation and spaced apart from one another a distance A/2, A 2 pi / Delta Beta , where Delta Beta is the phase constant difference required to satisfy the relationship Beta 1 + Beta 2 Beta 3 + Delta Beta , Beta 1, Beta 2 and Beta 3 being, to a first order of approximation, the phase constants respectively of the wave energy. In other embodiments, the spaces between the strips are filled with materials whose nonlinear optical coefficient is either zero or different from the nonlinear optical coefficient of the spaced apart strips. The filling material&#39;&#39;s index of refraction is substantially equal to that of the spaced apart strips.

United States Patent Yariv et al.

THIN FILM WAVEGUIDE WITH A PERIODICALLY MODULATED NONLINEAR OPTICALCOEFFICIENT Inventors: Ammon Yariv; Sasson R. Somekh,

both of Pasadena, Calif.

Assignee: California Institute of Technology,

Pasadena, Calif.

Filed: May 18, 1973 Appl. No.: 361,536

US. Cl. 307/88.3, 321/69 R, 350/96 WG Int. Cl. H03f 7/04 Field of Search307/883; 321/69 R;

References Cited UNITED STATES PATENTS 5/1968 Bloembergen 307/883Primary Examiner-Herman Karl Saalbach Assistant EkamirierDarwin R.Hostetter Attorney, Agent, or FirmLindenberg, Freilich, w s esmfiese..ie n n 57 ABSTRACT A phase matched thin film waveguide supportive ofelectromagnetic wave energy having angular frequencies is disclosed. Inone embodiment, the waveguide includes a sequence of strips each oflength A/2 in the direction of wave energy propagation and spaced apartfrom one another a distance A/2, A 21r/AB, where AB is the phaseconstant difierence required to satisfy the relationship 3, B [3 AB, 6,,B and [3 being, to a first order of approximation, the phase constantsrespectively of the wave energy. In other embodiments, the spacesbetween the strips are filled with materials whose nonlinear opticalcoefficient is v either zero or different from the nonlinear optical co-12 Claims, 4 Drawing Figures UTILIZATION DEVICE THIN FILM WAVEGUIDE WITHA PERIODICALLY MODULATED NONLINEAR OPTICAL COEFFICIENT ORIGIN OFINVENTION The invention herein described was made in the course of orunder a contract with the Office of Naval Research and Department of theArmy.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates tothin film devices, and more particularly, to aparametric device with a thin film waveguide whose nonlinear opticalcoefficient is periodically modulated along the direction of wave energypropagation for phase matching purposes.

2. Description of the Prior Art In recent years, considerable attentionhas been given to the field of thin film optics. One of the mostimportant features of a thin film guide is that due to its very smallwaveguide cross section area, small input power can cause very largeoptical power densities, so that nonlinear optical interactions becomevery efficient. Such interactions can be used to produce second harmonicgeneration, parametric oscillation and frequency up conversion.Unfortunately, many materials which have nonlinear optical propertiescannot be used directly since therein thewaves involved in the nonlinearinteraction are not phase matched, i.e., they travel with velocitieswhich cause destructive interference. Thus, in order to use suchmaterials, they have to be phase matched, so that they generate newwaves, generally thought of as spatial harmonics, with new phasevelocities for proper nonlinear optical interaction. Some materials are.phase matchable by a well known birefringent technique. However, somethin film materials'such as GaAs and 'GaP which possess high nonlinearoptical coefficients are not phase matchable by the conventionalbirefringent technique.

In US. Pat. No. 3,619,796, techniques other than the birefringenttechnique for producing phase matching in a thin film waveguide aredisclosed. One of the techniques consists of placing a-fixed gratingupon the surface of the thin film guide. A quantitative analysis ofthe-disclosed arrangement reveals that the power conversion efficiencyfor a given length of such a guide is quite small. Therefore, eventhough phase matching is theoretically achievable with the grating, theusefulness of the guide as part of a parametric device is quite limited.Also, the need for a separate mechanical grating on the thin film guideis quite undesirableboth from overall size and cost points of view.

OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of thepresent invention to provide a new thin film waveguide in which phasematching is achieved by a novel arrangement.

Another object of the invention is to provide a thin film waveguide withhigh conversion efficiency.

A further object of the invention is to provide a new thin film guidefor use as part of an optical parametric device with unique nonlinearoptical properties along the propagation direction.

These and other objects of the invention are achieved by providing athin film guide in which phase matching is achieved by periodicallymodulating its nonlinear oprection of wave energy propagation. Theperiod A is chosen so that AB 21r/A, where AB is the phase constantdifference required to satisfy the phase relationship ,B, +13 3;, +AB,where [3 B and B are to a first order of approximation of the phaseconstants, respectively of electromagnetic wave energy which issupportive by the guide with angular frequencies m m and (0 where w, m m

Designating the length of the first section of period A by d and thesecond section period by d where A d d,,, the nonlinear opticalcoefficient of the guide is periodically modulated by constructing it sothat along each length d it exhibits a high nonlinear opticalcoefficient and along each length d,, the nonlinear optical coefficientis different from that of the coefficient in the preceding section ofthe period. In one embodiment, the periodic modulation of the nonlinearoptical coefficient is realizable by forming the guide as a sequence ofspaced apart strips each of length d,, and of a materialeg, GaAs in aform, e.g., crystalline with a high nonlinear optical coefficient. Thestrips are spaced apart a distance d,,, wherein the nonlinear opticalcoefficient is zero. Thus, over each period A, the nonlinear opticalcoefficient varies between the nonlinear optical coefficient value ofthe strip and zero. The guide may also be formed as a sequence ofadjacent strips in which the odd strips have a length of d and anonlinear optical coefficient which is different from the nonlinearoptical coefficient of the even strips, which have the length of d,,.Theoretically, optimum performance is achieved when the polarity of thenonlinear optical coefficient of the evenstrip is opposite the polarityof the nonlinear optical coefficient of the odd strips.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will best be understood from thefollowing description y when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS 'FIG. 1 is an isometric view of asecond harmonic generator incorporating the novel waveguide of thepresent invention; g

FIG. 2 is a partial side view of another embodiment of the invention;and

FIGS. 3 and 4 are diagrams useful in analyzing the performance of thewaveguide of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventionwill be described in connection with second harmonic generation.However, as will be apparent to those familiar with the art, the novelphase matching technique employed herein is applicable for parametricoscillation or frequency upconversion. Attention is directed to FIG. Iwherein numeral l0 designates a waveguide structure consisting of asubstrate 11 which supports on the top thereof a thin film waveguidingstrip 12 hereafter referred to as the thin film guide. Electromagneticwave energy desig nated by dashed line 14, at a frequency f from asource 15 is directed to the thin film guide I2 which is phase 7 matchedin accordance with the present invention so that it produces an outputat a frequency 2f which is received by a utilization device 16. Theindices of refraction of the thin film strip 12 and the substrate 11 aredesignated n and n respectively, n n so that the electromagnetic wavepropagation is effectively confined to the thin film guide 12.

As previously pointed out in the present invention, phase matching isachieved by periodically modulating the nonlinear optical coefficient orproperties of the thin film guide 12 with a period of length A along thepropagation direction, designated as axis z in FIG. 1. The period A ischosen so that AB 21r/A, where AB is the phase constant difference whichhas to be made equal to zero for proper phase matching.

In one embodiment of the invention, as shown in FIG. 1, the periodicmodulation of the nonlinear optical coefficient is achieved by formingthe thin film guide 12 as a sequence of strips. In FIG. 1, only sixstrips are shown and are designated by numerals 12 12 although inpractice, the number of strips is much larger than six. The strips arearranged so that over any period of length A, the guide 1 includes onestrip, such as 12 of length d,, which has a high nonlinear opticalcoefficient and one strip, such as 12, of length d,, A d,, which doesnot have a nonlinear optical coefficient or whose nonlinear opticalcoefficient differs from that of strip 12 For explanatory purposes, itis assumed that the nonlinear optical coefficient of strips 12 12, and12 is zero.

Stated differently, the guide 12 is formed as a sequence of strips inwhich every other strip, e.g., each odd strip (12,, 12 etc.) possesses ahigh nonlinear optical coefficient while each even strip (12 12,, etc.)which occupies the space between two strips with the high nonlinearoptical coefficients is characterized by the absence of nonlinearoptical coefficient. Thus, in the direction of propagation z, thenonlinear optical coefficient of the guide 12 varies periodically from ahigh value in each odd strip to zero in each even strip. One way ofimplementing such a guide is'to form the odd strips from an appropriatematerial in a form with a high nonlinear optical coefficient, e.g., GaAsin crystalline form, wjile the even strips consist of the same materialin the amorphous or polycrystalline form. Such a guide of a thickness of20 microns p.) and a periodicity A 180g, provided very satisfactoryphase matching when used as a second harmonic generator providing anoutput signal at a wavelength of 5.3g in response to an input signalwith a wavelength of lO.6p..

If all the strips have the same index of refraction, the length of allthe strips is substantially the same, i.e., d,, d,,. If, however, theindex of refraction of the two kinds of strips is not the same, thelength of the two kinds of strips may not be the same. A zero orderapproximation for the period A is given by:

where AB and A3,, are the phase constant differences in the two kinds ofstrips defined by:

The same phase matching effect is achievable by eliminating the evenstrips thereby forming the guide 12 only as a sequence of strips eachwith a high nonlinear optical coefficient, with the strips being spacedapart from each other a distance equal to each strip length as shown inFIG. 2. Therein. the strips, each (1,, long, are designated by l2a-l2cand the spaces. each d,, long in the z direction, are designated by12d-12f. Again A d d,,.

Since the thickness of the guide 12 is the x direction is very small,generally in the order of a few microns, the guiding layer of thesubstrate 11 is readily accessible from the top surface of thin filmguide 12. Thus, one technique of forming the guide 12 is to first growit on top of the substrate 11 as a single thin film crystal andthereafter use an ion milling technique, which is known, to form thespaces l2d-12f as a series of grooves normal to the propagationdirection. The grooves may be sputter-filled with the amorphous, orpolycrystalline form of the film material to form the even stripsherebefore discussed in connection with FIG. 1.

The grooves may also be filled to form the even strips with somematerial other than the film material which does not possess a nonlinearoptical coefficient or with a material whose nonlinear opticalcoefficient differs from that of the strips formed from the originalthin film deposited on the substrate, and one with an index ofrefraction which is similar to that of the film material. Theoretically,the effect of the periodic modulation of the nonlinear opticalcoefficient of the guide 12 may be increased by forming the guide as asequence of strips as shown in FIG. 1 in which all strips have anonlinear optical coefficient except that the odd and even strips in thesequence have nonlinear optical coefficients of opposite polarities.Thus, instead of filling the grooves (or spaces l2d-l2f) with a materialin a form which does not possess a nonlinear optical coefficient theymay be filled with a material in a form which has a nonlinear opticalcoefficient of a polarity opposite to the polarity of the nonlinearoptical coefficient of the odd strips. For proper operation, it isdesirable that the top surface of the thin film guide 12, whether or notthe grooves are filled, be smooth to insure constant guide thickness.

The advantages of the period modulation of the nonlinear opticalcoefficient in accordance with the present invention, to achieve phasematching may better be appreciated by directing attention to thefollowing analysis.

Let us consider, for the sake of simplicity, a second harmonicgeneration in the dielectric waveguide shown in FIG. 3, in whichelements previously referred to are designated by like numerals. Theelectric field of the nth TE guided mode, as an example, is given by thefollowing expression:

u w (X: n n w PlK i H w The propagation phase constant [3,, as well asthe lateral mode profile where W is the width of th waveguide in the ydirection. [3,, varies between the bulk guide and substrate wave numbers(3) where k0 E 211/), is the free space wave number. For large t andsmall mode number [3,, approaches the upper limit, while the lower limitis approached by reducing the thickness t or choosing a higher numbermode. The electric field of the second harmonic mth mode is givensimilarly:

and the value for B,,, is confined between the two limits The secondharmonic polarization generated by the fundamental En'"(x, z, t) istaken as P (x, z, lf) .w.( )(A nf Vl otzdlF xp [iQag 2 1.;)] H (e) whered (x) is the appropriate bulk nonlinear tensor element. Thispolarization drives the second harmonic radiation, thus the rate ofgrowth of the average power in the mth second harmonic mode is given by:

Equation (8) readily gives the two requirements needed'for effectivenonlinear interaction. The first requirement is the well known phasematching condition which arises from the need to eliminate theoscillating exponential term in the differential equation (8). Thecondition is:

The second requirement for a high rate of second harmonic power growthis a large value for the overlap integral of the fundamental intensitymode profile and the second harmonic field profle in equation (8).

Under certain conditions, namely, n (ai) n (2w) it is possible tocompensate for the normal dispersion of th material and to phase matchby choosing the right thickness t and the mode numbers n and m. However,

this usually requires accurate control of the thickness and for n minvolves a large decrease in the magnitude of the overlap integral( 10)thus reducing the effective nonlinear coefficient for the interaction.

To overcome the problem of AB 0 in a thin film structure, let usconsider the waveguide 12 shown in F IG. 4. The nonlinear coefficient ofthe thin film guide 12 is modulated periodically with a period A, whilethe index of refraction is assumed to remain unchanged. To analyze thenew situation we note that the nonlinear coefficient d in (8) is now afunction of 2 as well as of x. We limit ourselves to the case where thefundamental and the second harmonic are well confined zero order modes.This makes it possible to neglect the x dependence of d so that (8) canbe written as if the spatial modulation period A is chosen equal to21r/A AB,

where d is the original nonlinear coefficient of the guide 12, and wherethe period A is chosen so that i2) is satisfied. It should be pointedout that d,, and d as shown in FIG. 4 represent the nonlinearcoefficients of adjacent strips and not their relative lengths along thez axis. Their lengths can be thought of as (1,, and (1,, where d,, a',,A. The Fourier expansion of this rectangular form nonlinear coefficientis:

Si (1 ml Using (14) and (12) in (l l) and keeping the synchronous termonly leads to The modes overlap integral reaches an optimum value whenthe modes are well confined and equals approximately r Using (2) toexpress A0 and A0 in terms of the respective mode powers (l5) begg ng Asimple manipulation of 16) where and B 1 are assumed to be equal to thebulk propagation constants gives in the nondepleted pump approximation:

This result is of a form identical to the bulk interaction except thathere the effective nonlinear coefficient is The conversion efficiencyfrom w to 2m is seen to be proportional to the mode power dens ity P/Wt. Since W and t can be made comparable to A this power density canbecome very large even for small power input. The penalty for modulatingd in order to phase match is a reduction of the effective nonlinearcoefficient by a factor of l/rr.

A physical picture of the way in which the spatial modulation of dovercomes the problem of phase at hin is the following (b9925 i the e rae second harmonic wave and the second harmonic polarization driving itdrift gradually (with distance) apart in phase. When ABz 1r theaccumulated phase shift is 1r/2 and power begins to flow back from thesecond harmonic to the fundamental. This happens after one coherencelength 1 E 1r/A/3. By having dNL equal to zero between z=l,- and 2 21the reversal of power flow is prevented. By 1 21, the accumulated phaseshift has returned to the favorable region (-l/21r d) l/2rr) and thenonlinear interaction is turned on (dnz 0) again. The reduced value ofd, as given by (18) refiects the fact that not all the physical lengthof the structure partakes in the interaction.

The practical implementation of the modulation of d seems feasible withpresently available techniques, since the guiding layer extends only afew microns below the surface and therfore is readily accessible. Oneapproach is to use ion-milling to fabricate a series of grooves normalto the propagation direction in a single crystal thin film guide andthen sputter-fill the grooves with a polycrystalline form of the filmmaterial (for which d 0) or with someother material with a similar indexof refraction.

It should be clear that a reversal of the sign of d in the second halfof each period results in the doubling of the value of d,.,,. This isachievable by forming guide 12 so that each odd strip has a nonlinearoptical coefficient of one polarity and each even strip, a nonlinearoptical coefficient of an opposite polarity.

It should be apparent that although the invention has been described inconnection with second harmonic generation, the teachings are equallyapplicable for phase matching the thin film guide for use in aparametric oscillator or a frequency up converter.

in summary the present invention provides a new phase matched thin filmguide for use in any parametric device. The thin film guide issupportive of electromagnetic wave energy having angular frequencies(U1, m and (0;, where w, w: w When used as a second harmonic generatorco =ai so that w =2w B and [3 are to a first order approximation. thephase constants, respectively, of the wave energy. The thin film guideis formed so that its nonlinear optical coefficient is periodicallymodulated over each period length A, where A, 3= 21r/A, and AB is thephase difference required to satisfy the relationship ,8, ,8 8;, AB.

The periodic modulation is achieved by forming the guide as a sequenceof strips each of length d in the direction of propagation and of aselected material in a form, e.g., crystalline, having a large nonlinearoptical coefficient. These strips are spaced apart a distance d,, whereA d,, d,,. The strips are formed by first depositing a thin film incrystalline form on a substrate and thereafter milling grooves of width(1,, spaced apart a distance d,,. The groove spaces when unfilledexhibit a nonlinear optical coefficient of zero in the direction ofpropagation. Thus, over each period A, the nonlinear optical coefficientis high as represented by the strips nonlinear optical coefficient andzero in the following space. If desired, the spaces may be filled with amaterial whose nonlinear optical coefficient is zero.

ln practice, the phase matched guide can be formed of one material, withthe strips in crystalline form and the grooves filled with the samematerial in amorphous or polycrystalline form. The grooves can also befilled with a different material having a zero nonlinear opticalcoefficient, or having a nonlinear optical coefficient significantlydifferent from the nonlinear optical coefficient of the first formedstrips.

ln any of these cases, the guide can be thought of as formed of anon-uniform material. It is clearly nonuniform when the grooves arefilled by a material dif ferent from that of the strips, which areformed from the originally deposited thin film. It is also of nonuniformmaterial even if the grooves are filled with the same material as thestrips since the latter are in crystalline form and the grooves arefilled with the material in other than crystalline form with a zerononlinear optical coefficient. Even when the grooves are not filled withany material, the guide, consisting of only spaced apart strips, can bethought of as being of a nonuniform, namely non-continuous, material. Itis clear that when the spaces consist of any material in a form so thatit exhibits a nonlinear optical coefficient of a polarity opposite thepolarity of the nonlinear optical coefficient of the original strips,the guide is of a nonuniform material.

Although particular embodiments of the invention have been described andillustrated herein, it is recog nized that modifications and variationsmay readily occur to those skilled in the art and consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:

l. A parametric device including:

a non-uniform nonlinear thin film waveguide supportive ofelectromagnetic wave energy having angular frequencies (0 m and m wherew, m (0 said waveguide comprising a substrate defining a top surface andat least a first group of spaced apart strips of a first material in aform having a nonlinear optical coefficient on the top surface of saidsubstrate, each strip having a length d,, in the direction of energypropagation, with adjacent strips of each said first group being spacedapart a distance d,,, where d,, A and A 21r/AB, AB being the phasedifference required to satisfy the phase relationship ,8, B ,8 AB and[3,, ,8 and B are to a first order of approximation. the phase constantsrespectively of said wave energy, the indicies of refraction of saidstrips and said substrate being definable as n. and n where n 2. Theparametric device ao described in claim 1 wherein said waveguide furtherincludes a second group of spaced apart strips on the top surface ofsaid substrate, each of length d,, with said first and second groups ofstrips being in one sequence of strips in which the odd strips consistof said first group and the even strips consist of said second group,each strip in said second group being of a material and in form which ischaracterized by the absence of a nonlinear optical coefficient.

3. The parametric device as described in claim 2 wherein said strips ofsaid second group are of said first material in a form different fromthe form of the strips in said first group.

4. The parametric device as described in claim 2 wherein said strips ofsaid second group are of a material different from said first material.

5. The parametric device as described in claim 1 wherein said waveguidefurther includes a second group of spaced apart strips on said topsurface of said substrate, each strip being of a length d,, with saidfirst and second groups of strips being in one sequence of strips inthe'direction of energy propagation, with the odd and even strips in theseqence consisting of said first and second groups of strips,respectively, each .even strip being of a material and in a form whichis wherein the nonlinear optical coefficient of each even strip ischaracterized by a polarity which is opppsoite the polarity of thenonlinear optical coefficient of the odd strips.

8. The parametric device as described in claim 1 wherein A is in therange of one hundred to a few hundred microns.

9. The parametric device as described in claim 8 wherein the thicknessof said strips in a direction perpendicular to the direction of energypropagation is of the order of not more than several tens of microns.

10. The parametric device as described in claim 2 wherein A is in therange of one hundred to a few hun-' dred microns.

11. The parametric device as described in claim 5 wherein A is in therange of onehundred to a few hundred microns.

12. A parametric device including: a non-uniform nonlinear thin filmwaveguide supportive of electromagnetic wave energy having angularfrequencies 0),, m and (0 where to, m (0 said waveguide comprising asubstrate defining a top surface and a single group of spaced apartstrips of a first material in a form having a nonlinear opticalcoefficient, each strip having a length d,, in the direction of energypropagation, with adjacent strips of said first group being spaced aparta distance d,,, where d,, A and A 2rr/AB, AB being the phase differencerequired to satisfy the phase relationship B B [3 AB and [3,, B and Bare to a first order of approximation, the phase constants respectivelyof said wave energy, said strips and said sub strate being characterizedby indicies of refraction definable as n and n respectively, where nn;,.

Patent No.

Inventor(s) 3, 842 289 Dated October 15, 1974 Amnon Yariv, et al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 3 line line line line 60 change "6 A to -B 8 Column 4 v I line 3,change "is" to --in-- l to line 51, in equation (1) change "a?" to "-5line 65, change "th" to --the-' Column 5 2w line 10, i equation (4)chang "e to E and change 'line 2%, in equation (6) change last term '2ez" to Column 6 Column 7 I I 2B 2-- w 0) Line 4%, in equation (9) change"2e to --2B line 57 change "n (m) n (2m)"' to --B w n (2w? line 1,change "A" to -A-- In line of text between equations (11) and (12)change "A" to --A-- line 32, in equation 13, change "6.5" to --d-- 7line 38, change "d to --d;--

. I w line 1, change "8 to --Bg5 line 1, change "a to 0 line 65, change"00 2w B to --w 2w B ,B

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, DatedOctober 15,

Inventor(s) m Variv ji- :1]

It is certified that error appears in the aboveidentified patent andthat said Letters Patent are hereby corrected as shown below:

Continued--- 2 Column 8 7 line 62, Claim 1, before "said" delete "each"line 63, in the equation, change "d to --d d Column 9 line 2, Claim 1,change "n to n n3 line 3, Claim 2, "ac" shou d e -as-- line 10, after"in" insert -a-- line 35, Claim 7, "a0" should be --as-- Column 10 line2, Claim 7, "opppsoite" should be -.-opposite-- line 28, Claim 12,change "d db=A" to :'-d d A- line 34, Claim 12, change ,"n n to --n nSigned and sealed this 11th day of March 1975.

(SEAL) Attest:

- C. MARSHALL DANN RUTH C. MASON Commissioner of Patents AttestingvOfficer and Trademarks

1. A parametric device including: a non-uniform nonlinear thin filmwaveguide supportive of electromagnetic wave energy having angularfrequencies omega 1, omega 2 and omega 3 where omega 1 + omega 2 omega3, said waveguide comprising a substrate defining a top surface and atleast a first group of spaced apart strips of a first material in a formhaving a nonlinear optical coefficient on the top surface of saidsubstrate, each strip having a length da in the direction of energypropagation, with adjacent strips of each said first group being spacedapart a distance db, where da + db A and A 2 pi / Delta Beta , DeltaBeta being the phase difference required to satisfy the phaserelationship Beta 1 + Beta 2 Beta 3 + Delta Beta and Beta 1, Beta 2 andBeta 3 are to a first order of approximation, the phase constantsrespectively of said wave energy, the indicies of refraction of saidstrips and said substrate being definable as n2 and n3 where n2 < n3. 2.The parametric device ao described in claim 1 wherein said waveguidefurther includes a second group of spaced apart strips on the topsurface of said substrate, each of length db with said first and secondgroups of strips being in one sequence of strips in which the odd stripsconsist of said first group and the even strips consist of said secondgroup, each strip in said second group being of a material and in formwhich is characterized by the absence of a nonlinear opticalcoefficient.
 3. The parametric device as described in claim 2 whereinsaid strips of said second group are of said first material in a formdifferent from the form of the strips in said first group.
 4. Theparametric device as described in claim 2 wherein said strips of saidsecond group are of a material different from said first material. 5.The parametric device as described in claim 1 wherein said waveguidefurther includes a second group of spaced apart strips on said topsurface of said substrate, each strip being of a length db with saidfirst and second groups of strips being in one sequence of strips in thedirection of energy propagation, with the odd and even strips in theseqence consisting of said first and second groups of strips,respectively, each even strip being of a material and in a form which ischaracterized by a nonlinear optical coefficient which is different fromthat of the odd strips.
 6. The parametric device as described in claim 5wherein each even strip is of a material and form characterized by anonlinear optical coefficient which is other than zero.
 7. Theparametric device ao described in claim 6 wherein the nonlinear opticalcoefficient of each even strip is characterized by a polarity which isopppsoite the polarity of the nonlinear optical coefficient of the oddstrips.
 8. The parametric device as described in claim 1 wherein A is inthe range of one hundred to a few hundred microns.
 9. The parametricdevice as described in claim 8 wherein the thickness of said strips in adirection perpendicular to the direction of energy propagation is of theorder of not more than several tens of microns.
 10. The parametricdevice as described in claim 2 wherein A is in the range of one hundredto a few hundred microns.
 11. The parametric device as described inclaim 5 wherein A is in the range of one hundred to a few hundredmicrons.
 12. A parametric device including: a non-uniform nonlinear thinfilm waveguide supportive of electromagnetic wave energy having angularfrequencies omega 1, omega 2 and omega 3 where omega 1 + omega 2 omega3, said waveguide comprising a substrate defining a top surface and asingle group of spaced apart strips of a first material in a form havinga nonlinear optical coefficient, each strip having a length da in thedirection of energy propagation, with adjacent strips of said firstgroup being spaced apart a distance db, where da + db A and A 2 pi /Delta Beta , Delta Beta being the phase difference required to satisfythe phase relationship Beta 1 + Beta 2 Beta 3 + Delta Beta and Beta 1,Beta 2 and Beta 3 are to a first order of approximation, the phaseconstants respectively of said wave energy, said strips and saidsubstrate being characterized by indicies of refraction definable as n2and n3 respectively, where n2 < n3.